1. Field of the Invention
The present invention relates to devices for sensing moisture content in soils and to thereby monitor it and/or control the supply of water to desired areas in agricultural and horticultural situations and is the culmination of a wide experience with a commercially practical sensor.
2. Description of Related Art
U.S. Pat. No. 4,513,608 discloses one form of sensing device, hereafter referred to as Type xe2x80x9cAxe2x80x9d, having two separate porous zones one with relatively large pores which approximate the range of field soil pores, the second with fine pores. A pair of electrodes in each zone thereby forms a current path through the respective porous zones. In use the device is placed in an in ground position such that ground moisture is in intimate contact with it to maintain a reliable hydraulic and chemical equilibrium with the porous zones. The electrical resistance in the current paths through the respective porous zones is then used to indicate in ground soil moisture levels. This device has proven to remain accurate and reliable in practice and a large number have been applied in commercial use however it requires costly ceramic materials, is labour intensive and is difficult and expensive to manufacture.
There have been many other moisture content sensor designs proposed using various techniques including electrical resistance, capacitance, reflectrometry and thermal diffusivity, all of which have introduced their own problems when operating in such a physically, chemically and biologically hostile environment as prevails in field soils.
As examples of electrical resistance, U.S. Pat. Nos. 4,892,113, 4,879,498, 4,852,802, 4,796,654, 4,785,843, 4,718,446 and 4,693,419 all relate to bare electrodes directly in contact with the soil which sense an increase in electrical resistance as the soil dries and a decrease as it becomes moist. This principle, referred to as Type xe2x80x9cBxe2x80x9d, is useful as long as other factors remain stable however the wide swings in the electrical conductivity (EC) of the soil solution which drops with plant uptake, rainfall, irrigation or increase in soil temperature and rises with fertilisation, water borne salts or decrease in soil temperature, can produce resistance changes many times greater than the wet to dry changes masking them to an unacceptable degree also it experiences difficulty in maintaining a reliable area of contact with representative soil, make it quite impractical as an accurate moisture monitor.
U.S. Pat. Nos. 4,952,868, 4,216,789 and 4,531,087 introduce a moisture permeable retainer for a controlled media in which the moisture resistance is determined. Referred to as Type xe2x80x9cCxe2x80x9d, the retainer presents problems of hydrophobic barrier to moisture movement into the media. These types also do not include solution conductivity compensation to solve the problems of Type xe2x80x9cBxe2x80x9d even though a subsequent refinement to U.S. Pat. No. 4,531,087 introduces a pellet of a low solubility salt to ionise and mask the soil solution conductivity effect. This modification, referred to as Type xe2x80x9cDxe2x80x9d, is only partially successful and the pellet progressively dissolves.
U.S. Pat. No. 4,137,931 utilising a Type xe2x80x9cCxe2x80x9d sensor introduces a second Type xe2x80x9cCxe2x80x9d sensor in the irrigation water supply and again in combination alongside the first sensor aimed at control of leaching and not proposed or modified or connected as a means of compensating the first sensor for change in EC of the soil solution. This sensor combination is referred to as Type xe2x80x9cExe2x80x9d.
U.S. Pat. No. 4,561,293 combines a Type xe2x80x9cCxe2x80x9d retainer like design with a fine media resistance element to compensate for changes in EC of the soil water solution. This design referred to as Type xe2x80x9cFxe2x80x9d has basic similarities to Type xe2x80x9cAxe2x80x9d but practical design problems associated with maintaining moisture movement through the retainer, blockage of its necessarily small pores, the small area of its soil moisture contact and no electrical isolation from the soil potentials prevented reliable operation except under laboratory conditions.
The foregoing description of prior art devices should not be taken as indicating any of the devices are necessarily part of the general knowledge in this industry.
The objective of the present invention is to provide a new moisture sensing device, and preferably associated equipment, that will enable the device to operate accurately enough to control irrigation efficiently over a wide range of soil types and soil solution electrical conductivities that will avoid costs associated with sensors of the type described in U.S. Pat. No. 4,513,608.
In accordance with the present invention, there is provided a soil solution sensing device including a body formed from porous material which, in use, is placed in intimate hydraulic conduction with soil in an area under surveillance, said body forming a first zone having pore sizes that approximate a range of pores in typical soils, a second zone being formed within said first zone having pores arranged to be small enough to remain hydrated at matric tensions of the soil solution significantly above those which cause the first zone to dehydrate, said first zone containing a first electrode and said second zone containing a second electrode, each of which are paired with an electrode common to both said zones whereby, as the matric tension of the soil solution in the soil in the surveillance area increases, the pores within the first zone progressively dehydrate causing electrical resistance between the first electrode and the common electrode to progressively increase while the pores in the second zone remain hydrated such that the electrical resistance between the second electrode and the common electrode changes only in response to conductivity of the soil solution in the pores of the second zone.
In pursuing the wide commercial practical success of the Type xe2x80x9cAxe2x80x9d sensor design and in addressing its cost problem, the new sensor has been designed to achieve an improved performance at a fraction of its cost. Such features of the Type xe2x80x9cAxe2x80x9d sensor as its ability to operate over the wide range of soil solution electrical conductivities (EC) and in a preferred aspect, its technique for electrical isolation from the soil have been improved and retained.
The new design conveniently provides a single electrode in each zone, each of which pair with a single common electrode which may be designed as an electrically isolating shield between the respective zones. This reduces the size of the ceramic block and preferably introducing a void between the first zone electrode and the common electrode, the matric tension to electrical resistance response can be biassed. Lower cost body materials may also be used. The design of the common and preferably shielding electrode eliminates one electrode, simplifies assembly and enables a reduction in sensor field cables from 4 cores to 3 cores.
In preferred aspects, this technique has been further refined by developing a concept for a soil type selector to optimise setting up of the device according to soil type. This present design eliminates most of the high cost components of Type xe2x80x9cAxe2x80x9d and the hydraulic barriers of Type xe2x80x9cCxe2x80x9d whilst retaining the EC compensation and the magnetic isolation of Type xe2x80x9cAxe2x80x9d and by introducing a convenient means of soil type selection.